It is the handiwork of the United Coalition of Reason, a Washington, D.C.-based group formed three years ago with money from an anonymous donor. The coalition has paid for billboards and bus ads in Washington and 31 states.

The timing of its arrival in San Antonio creates a contrast on the roads. A grass-roots Christian billboard campaign here called “Think God” has been putting up billboards across the area in recent months.

The atheist digital billboard, 14 feet by 48 feet, cost $5,000, said Fred Edwords, the coalition's national director. It displays the coalition's website and aims to connect nonbelievers and urge them to emerge from isolation and fear, he said.

“There's a lot of momentum, and new (atheist) groups are forming all the time,” he said. “The task here at hand isn't to convince anyone to change their views or give up their religion. It's to (support) all the people who already think like we do and get them into a local group.”

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The number of U.S. adults who say they are atheist, agnostic or unaffiliated has increased in recent years, with the proportion now 16 percent, according to the Pew Research Center. Other surveys put it at 12 percent.

Growing assertiveness in recent years by both nonbelievers and religious conservatives has intensified debates and legal battles over the line between church and state.

“That is quite an expensive way of sharing their thoughts,” Metzger said. “I think we've lived long enough to see numerous times when the ‘God is dead' ideas have been thrown out.”

Billboards have long been in use for religious messages, broad and narrow.

A California minister last year used them to publicize his calculation of a May 21 date for the second coming of Christ. The Church of Jesus Christ of Latter-day Saints has a billboard campaign here and in other cities to depict Mormons as a diverse group of mainstream Americans.

The first four “Think God” billboards that went up here in December 2010 were the brainchild of Michael Hoover, a commercial real estate agent. He said he picked up the phrase from a character in the 1980 George Burns movie, “Oh God! II” and thinks God led him to use it for a simple, nondenominational campaign.

The group expanded the effort in March to a dozen billboards, including one each in Comfort, the Canyon Lake area and Round Rock. The campaign launched its website, www.thinkgod.us, on Saturday and is already attracting funding to spread the billboards nationwide, they said.

“When you see the words, ‘Think God,' they are a command to the brain,” Hoover said. “You can't not think about anything else, whether you're an atheist or a believer.”

Interviewed Tuesday, the group said they hadn't seen the atheist billboard. It links to a website for the San Antonio Coalition of Reason, with contact information about its six member groups in South Central Texas. Organizers said their donor is staying anonymous to avoid solicitation but that his support is helping to remove longstanding stigma.